The invention belongs to the field of nondestructive testing, and relates to a ray tracing type ultrasonic Lamb wave defect tomographic imaging method, first, a to-be-measured material imaging area is divided into a plurality of small grids, and the same amount of emission EMAT (electro-magnetic acoustic transducer) and receiving EMAT are arranged on both sides of the area, then all emission ends are allowed to excite A0 mode Lamb wave, all receiving ends at the opposite side receive meanwhile, analysis results of all tested waveforms are extracted, the corresponding time is recorded, the slowness and defect distribution of every grids are determined by use of combined iterative reconstruction algorithm, the obtained defect distribution is used as a known condition for establishment of a RT extrapolation formula, by use of the extrapolation method, a ray path is gradually traced and corrected, and the ray path is gradually traced and corrected repeatedly until high imaging precision is obtained by calculation. The a ray tracing type ultrasonic Lamb wave defect tomographic imaging method avoids the problem of low imaging precision of a traditional TOF cross hole tomographic imaging method based on straight ray theory, is accurate, efficient and fast to calculate, provides an important basis for improving the defect imaging precision, and has broad application prospects.